I usually warm my frozen film for a couple of hours but never heard a suggestion for days before.

If you think about how wide and long the 70mm is, since it's not sealed completely, if any moisture is in it, and crystals in the emulsion form, and then are melted, they need time to evaporate out of the central core before being shot, if they are shot too soon, you run the risk of the emulsion being raised in the moist areas. That's my guess anyway.